For my campaign I want to use only Pandemonium and Gehenna for the lower planes, which means plenty of yugoloths and also howlers. There's always been plenty of talk of how the Monster Manual and Volo's Guide don't have enough monsters for higher levels, so it seems that for Mordenkainen's Tome they simply took the critters they still had to stat and assigned them higher CRs. Which does the job, but completely reshuffles the ranking of the relevant creatures. Looking at their stats in 2nd and 3rd edition and comparing how other creatures were converted in the Monster Manual, canoloths should be CR 4 and howlers CR 3, not CR 8. So I reworked them to match those CRs and also statted up piscoloths.

While trying to figure out what abilities a piscoloth should have I went back to the AD&D stats and realized that a piscoloth stripped of its yugoloth traits is a straight up chuul. For more than 18 years, I never realized that the chuul is a knockoff.
This did make the conversion fairly simple, though.

Actions
Multiattack: The piscoloth makes two pincer attacks. If the piscoloth is grappling a creature, the piscoloth can also use its tentacles once.
Pincer: Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage. The target is grappled (escape DC 15) if it is a Large or smaller creature and the piscoloth doesn't have two other creatures grappled.
Tentacles: One creature grappled by the piscoloth must succeed on a DC 14 Constitution saving throw or be poisoned for 1 minute. Until this poison ends, the target is paralyzed. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Teleport: The piscoloth magically teleports, along with any equipment it is wearing or carrying, up to 60 feet to an unoccupied space it can see.

I think the bonuses and DCs should all be correct, but if anyone wants to recheck that would help.